feat(metadata): SQLite backend, storage reconciliation and local defaults

- pluggable per-domain Repository interfaces; PostgreSQL and SQLite impls
- SQLite on modernc.org/sqlite (WAL, busy_timeout, foreign_keys, immediate tx)
- separate migrations/sqlite and db.MigrateSQLite; store.NewPostgres/NewSQLite
- cmd/reconcile compares object storage with the journal (report, then --delete)
- storage.Backend.List and photos.Repository.AllStorageKeys
- DB_BACKEND/SQLITE_PATH and STORAGE_BACKEND/FS_ROOT selectable; local defaults
  (sqlite ./galo.db, fs ./data)
- Makefile reconcile/reconcile-delete; docs and .env.example updated

backend/cmd/server/main.go

@@ -19,6 +19,7 @@ import (
 	"shared-album/internal/db"
 	"shared-album/internal/sse"
 	"shared-album/internal/storage"
+	"shared-album/internal/store"
 )
 
 func main() {
@@ -37,37 +38,65 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
-	migFS, err := fs.Sub(assets.Migrations, "migrations")
-	if err != nil {
-		slog.Error("migrations fs", "error", err)
-		os.Exit(1)
+	var (
+		repos  store.Repos
+		ping   func(ctx context.Context) error
+		closer func()
+	)
+	switch cfg.Database.Backend {
+	case "sqlite":
+		migFS, err := fs.Sub(assets.Migrations, "migrations/sqlite")
+		if err != nil {
+			slog.Error("migrations fs", "error", err)
+			os.Exit(1)
+		}
+		if err := db.MigrateSQLite(cfg.Database.SQLitePath, migFS); err != nil {
+			slog.Error("migration failed", "error", err)
+			os.Exit(1)
+		}
+		database, err := db.OpenSQLite(cfg.Database.SQLitePath)
+		if err != nil {
+			slog.Error("database open failed", "error", err)
+			os.Exit(1)
+		}
+		repos = store.NewSQLite(database)
+		ping = database.PingContext
+		closer = func() { _ = database.Close() }
+	default:
+		migFS, err := fs.Sub(assets.Migrations, "migrations")
+		if err != nil {
+			slog.Error("migrations fs", "error", err)
+			os.Exit(1)
+		}
+		if err := db.Migrate(cfg.Postgres.DSN(), migFS); err != nil {
+			slog.Error("migration failed", "error", err)
+			os.Exit(1)
+		}
+		pool, err := db.Connect(ctx, cfg.Postgres.DSN())
+		if err != nil {
+			slog.Error("database connect failed", "error", err)
+			os.Exit(1)
+		}
+		repos = store.NewPostgres(pool)
+		ping = pool.Ping
+		closer = pool.Close
 	}
-	if err := db.Migrate(cfg.Postgres.DSN(), migFS); err != nil {
-		slog.Error("migration failed", "error", err)
-		os.Exit(1)
-	}
-	slog.Info("migrations applied")
+	defer closer()
+	slog.Info("migrations applied", "db_backend", cfg.Database.Backend)
 
-	pool, err := db.Connect(ctx, cfg.Postgres.DSN())
-	if err != nil {
-		slog.Error("database connect failed", "error", err)
-		os.Exit(1)
-	}
-	defer pool.Close()
-
-	store, err := storage.Open(ctx, cfg)
+	blob, err := storage.Open(ctx, cfg)
 	if err != nil {
 		slog.Error("storage init failed", "error", err)
 		os.Exit(1)
 	}
-	if err := store.EnsureBucket(ctx); err != nil {
+	if err := blob.EnsureBucket(ctx); err != nil {
 		slog.Warn("could not ensure bucket", "error", err)
 	} else {
-		slog.Info("storage bucket ready", "bucket", store.Bucket())
+		slog.Info("storage ready", "location", blob.Bucket())
 	}
 
 	hub := sse.NewHub(cfg.SSE.ClientBuffer)
-	server := app.New(cfg, pool, store, hub)
+	server := app.New(cfg, repos, ping, blob, hub)
 
 	go server.StartWorker(ctx)